Polynya dynamics and export of dense water Results from the Storfjord laboratory Peter M. Haugan...

Polynya dynamics and export of dense water Results from the Storfjord laboratory

Peter M. Haugan1,2, Ragnheid Skogseth3, Ilker Fer2,1, Lars H. Smedsrud2,1

1Geophysical Institute, University of Bergen, Norway2Bjerknes Centre for Climate Research, Bergen

3University Centre on Svalbard

• Observed polynya opening and closing, salt rejection and build-up, outflow, entrainment and mixing from satellite and in situ data• Interannual variability• Future use of the ”laboratory”

www.gfi.uib.no, www.bjerknes.uib.no, www.unis.no

ProClim project web site www.noclim.org

Some referencesSkogseth, R., I. Fer and P.M. Haugan 2005. Dense-water production and outflow from an Arctic

coastal polynya in Storfjorden. In Helge Drange, Trond Dokken, Tore Furevik, Ruediger Gerdes and Wolfgang Berger (Ed.): The Nordic Seas: An Integrated Perspective, volume 158 of AGU Geophysical Monograph, pages 73-88. American Geophysical Union

Skogseth, R., P.M. Haugan and M. Jakobsson 2005. Watermass transformations in Storfjorden. Cont. Shelf Research 25, 667-695.

Skogseth, R., P.M. Haugan and J. Haarpaintner 2004. Ice and Brine Production in Storfjorden From Four Winters of Satellite and in Situ Observations. J. geoph. Res. 109, C10008, doi:10.1029/2004JC002384.

Fer, I., R. Skogseth and P.M. Haugan 2004. Mixing of the Storfjorden (Svalbard Archipelago) overflow inferred from density overturns. J. geoph. res. Vol. 109, C01005, doi:10.1029/2003JC001968.

Fer, I., R. Skogseth, P.M. Haugan and P. Jaccard 2003. Observations of the Storfjorden overflow. Deep-Sea Research Part I, Vol 50 (10-11) pp 1283-1303.

Fer, I. 2006. Scaling turbulent dissipation in an Arctic fjord. Accepted in Deep-Sea Res. II.Smedsrud, L.H. and R. Skogseth 2006. Field measurements of Arctic grease ice properties and

processes. Accepted in Cold Regions Science and Technology doi:10.1016/j.coldregions.2005.11.002

Smedsrud, L.H., Budgell, W.P, Jenkins, A.D. and Ådlandsvik, B. 2006 Fine scale sea ice modelling of the Storfjorden polynya. Accepted in Annals of Glaciology Vol. 44.

Haarpaintner, J. , J.-C. Gascard and P.M. Haugan 2001. Ice production and brine formation in Storfjorden. J. geoph. res. 106(C7) 14001-14013.

Haarpaintner, J., P.M. Haugan and J.-C. Gascard 2001. Interannual variability of the Storfjorden (Svalbard) ice cover and ice production observed by ERS-2 SAR. Ann. Glaciol. 33, 430-436.

Haarpaintner, J., J. O'Dwyer, J.-C. Gascard, P.M. Haugan , U. Schauer, and S. Østerhus 2001. Seasonal transformation of water masses, circulation and brine formation observed in Storfjorden, Svalbard. Ann. Glaciol. 33, 437-443.

Thanks also to Jørg Haarpaintner, student at UNIS 97/98, PhD at Univ. of Paris 2001

Storfjord

Arctic Ocean coastal polynyas

• supply the basins with densedeep water

• help maintaining the Arctic halocline

Exported dense water contributes to the meridional overturning circulation

UNISLongyearbyen

Storfjorden polynya

April 6 2001April 6 2001

Storfjorden• 190 km long, 190m deep • Cyclonic Coastal Current (East Spitsbergen C. – Sørkapp C.)

• Access to Atlantic Water and main shelf break• Strong tidal currents in northern sounds (4-5 m/s and 2-3 m/s, respectively)

• Area about 13103 km2 and volume of 850 km3

• Roughly 16% of the area is deeper than the 120m sill; the corresponding volume is 5% of the total

Formation of brine-enriched shelf water (BSW)

shelf water

Surface advection(Polar Front Water)

Surface advection through the sounds (Arctic)

D i f f u s i o n / c o n v e c t i o nD i f f u s i o n / c o n v e c t i o n

Sill77ºN

Advection from South

Out-flow

Pack IceFast Ice

B ri ne

re le as e d ur

in g fr ee zi n g

Storfjorden Polynya

shelf water

D i f f u s i o n / c o n v e c t i o nD i f f u s i o n / c o n v e c t i o n

Sill77ºN

Advection from South

Out-flow

Pack IceFast Ice

B ri ne

re le as e d ur

in g fr ee zi n g

Storfjorden Polynya

Brine enriched shelf water

Diffusion / convection

Sill77ºN

Advection from South (AW)

Out-flow

Pack IceFast Ice

e release du

ring freezin

Storfjorden Polynya

ERS-2 SAR Interpretation

(Modified from Haarpaintner, Gascard & Haugan, JGR, 2001)

Observed polynya widths during 2000 and 2001

Error bars show spread in widths from satellite data at different locations along polynya.

Note that polynya is defined here as both open water, frazil and thin ice.

Modelled polynya widths in fully drawn line.

Observed areal fractions• Fast ice (white)• Polynya (black)• Pack ice (grey)during 5 winters from ERS-2 SAR

November – May

Haarpaintner et al. (JGR, 2001): Winter 98

Haarpaintner et al. (Annals of Glaciol., 2001): Winters of 98 and 99

Skogseth et al. (JGR, 2004): 4 winters from 98-2001, revised model and 32 year simulation

Skogseth et al. (AGU Monograph, 2005): 5 winters including 2002

Simple polynya modelApproach:– Satellite observations are used to describe polynya

width defined by distance from shore to pack ice.– A wind driven polynya width model is matched to the

observations using winds from a nearby weather station. Opening and closing factors for the model are found.

– An accompanying wind and heat flux driven open water width model is used to distinguish open water (frazil ice) zone from thin ice zone within polynya.

– Total ice freezing within polynya region (and in pack ice/fast ice regions) over the whole Storfjord basin is computed.

[Nilsen, Weigel & Skogseth (in prep. 2006): 7 winters including 2004, show that opening factor is closely related to wind stress curl and regional ice conditions in the Barents Sea]

Polynya and open water width models

tUBAPWPW nnnnn )cos()( 0101

fnnnn h

hOWtUBOW 1)cos( 102

s 168.01.132

129.0 112

fnontntntnt A

hAHHAH

1,,1,1,,

Polynya width PW, Open water width OW (in polynya), Wind Un, B1 = 0.02, B2 = 0.04:

Frazil ice growth, Fnet from full heat budget

Growth of thin (Ht) and fast/pack ice (Hfp),: freezing-degree-seconds,hs: snow thickness (only for fast/pack ice)

Mass conservation of thin ice,Areas given as (Width) x (Length= 48 km)

A: open/close factor0: preferred directionn: actual wind direction

hc: collection thickness

hc = (1.0m + 0.1s·Un)/15

Salt release from modeled ice production and observed surface salinity

• Most in winter 2002

• Least in winter 1999

• About 1-1.5 Mt of released mass of salt each year

Assuming 69% immediate release during freezing and 10 % from aging while still in basin

Observed BSW salinity

Varies interannually by more than 1, winter 2002 is the most saline.

[Model also gives highest salt release in 2002.]

Strong northeasterly wind component

high polynya activity and ice production in Storfjorden

large ice transport from the Arctic into the Barents Sea

lower air temperature increases the ice growth and ice cover in the western Barents Sea

the surface water will be fresher and the water column more stable the following summer inhibit large production of BSW the next winter

High NAOHigh NAO

low polynya activity and ice production in Storfjorden

less ice transport from the Arctic into the Barents Sea

higher air temperature decreases the ice growth and ice cover in the western Barents Sea

the surface water will be more saline and the water column less stable the subsequent summer favors large production of high salinity BSW the next winter

Weak northeasterly wind component

Observations of mixing of the Storfjorden overflow

Hydrographic conditions (summer)• Fresh, cold melt water mixed

with Arctic water advected with coastal current down to 65-75 m.

• Stronger inflow of Atlantic water in the exchange zone with a core at 60-70 m depth.

• Overflow of BSW in the exchange zone. Overflow ceases during autumn.

• Census and overview of seasonal changes in Skogseth et al. (CSR, 2005)

May2001

Two layered overflow: May 2001• Identified as two

layers: 1) a lower layer (~15 m)

with relatively uniform vertical structure

2) an upper, thick mixing layer (~30 m) with larger vertical density gradient

• The widening of the lower layer is comparable to Ekman veering (friction from the bottom)

CTD and LADCP data covered nearly 37 km of the plume path with starting point about 68 km downstream of the sill

Survey-averaged profiles from 2002 Thorpe scale analysis

Mixing estimated from CTD data 2002: Thorpe scale analysis

Effect of stratification

Note: Exponent here -1.2 ± 0.3. Later study with microstructure measurements (Fer, 2006) gives -1.4 ± 0.2 and confirms validity of Thorpe scale method

Some conclusions from overflow studies

• Most of the overflow takes place on the western side of the sill entrance with a geostrophic transport of 0.01-0.21 Sv

• Transport doubled from 0.06 to 0.12 Sv along the observed path

• Observed core of the plume is captured by the predicted path from Killworth’s simple model (local equilibrium, constant Rio)

• Shear induced mixing at the dense plume and ambient water interface is responsible for the upper layer of the overflow.

• Bottom generated turbulence homogenizes the lower part of the plume and hence, is responsible for the structure of the lower layer of the overflow.

• The plume gains a considerable amount of heat from the overlying Atlantic Water. Heat flux estimates from finestructure compare well with the rate of change of heat in its core.

Summary

Storfjorden contributes 6-15% of total ice production and 3-20% of dense water formation in Arctic coastal polynyas:

• Proximity to Atlantic Water and main shelf break• Recurrent polynya• Time scale of BSW accumulation and flushout < 1 year• Quantifiable interannual variability -> ”Laboratory”

Simple approach provided fundamental understanding, but source->basin->export scheme has complexities.

Identified possibilities for feedback and oscillations involving wind, external ice conditions and source waters affecting total ice production, BSW volume and BSW salinity.

Quantified mixing processes affecting gravity current.

Further workPlans:Winter field work in 2006 addressing freezing/salt release

mechanism and ice thickness distributionSummer field work in 2006 addressing ambient mixing and

effects on overflow and gravity currentOutflow (bottom ADCP) moorings planned through 2008Coupled atmosphere-ice-ocean modelling in progress

Issues:Role of tides in addition to wind as forcing for polynya

opening (-> current meters near northern sounds)Internal cascading in the basin (-> distributed sensors)Maintaining time series of satellite data, CTD and other data

for improved understanding of interannual variability

Salt content in newly frozen sea ice

Lars H. Smedsrud taking samples of frazil ice

Winter field work continuing in 2006